1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the invention relates to a magnetic recording medium such as an audio cassette tape or a video cassette tape which is improved in heat resistance.
2. Description of Prior Art
As a magnetic recording medium (also referred to hereinafter as a magnetic tape) such as an audio tape or a video tape, a recording tape comprising a nonmagnetic support and a magnetic recording layer provided on the support which contains a ferromagnetic powder dispersed in a resin component (binder) is generally employed. Recently, the audio tape or video tape has been widely employed in the form of a cassette encasing the tape therein.
Previously, the audio tape or video tape encased in a cassette has been used or stored under mild ambient conditions with little temperature change such as in a room of a residence. However, as a stereophonic system in an automobile (i.e., car stereo) is developed, the audio tape is now frequently used or stored in an automobile. It is reported that the temperature inside of an automobile sometimes elevates to a very high level, for instance, up to 104.degree. C. when the automobile is allowed to stand for a certain period of time under exposure of sunlight in a summer season according to high-temperature or low-temperature exposure test of automobile components (JIS-D-0204). Further, when an audio tape installed in a cassette-type appliance with radio is used outdoors, the tape is sometimes exposed to a relatively high temperature. A video tape is also sometimes exposed to a high temperature when the tape is used outdoors with a handy video tape recorder or a video camera.
Most of the conventional magnetic recording tapes shrink in the longitudinal direction to show overall curling and/or shrink in the width direction to show partial curling when kept for a long period at a high temperature. Further, the conventional magnetic recording tape wound around a hub sometimes pushes the hub out when left to stand at high temperatures, because the wound tape shrinks to tighten the hub greatly. The magnetic recording tape suffering from such deformation shows extreme variation of output in the reproduction mode, and hence no normal reproduction can be done. Further, such deformed tape cannot run smoothly. The deformation of the magnetic recording tape is more frequently observed with respect to relatively long magnetic recording tapes for playing a long period of time such as audio cassette tapes of C-80, C-90 and C-120 types and video cassette tapes for long time play. In such long recording tape, the tape is wound over a hub in extremely many folds so as to deform or push the hub out more frequently.
Accordingly, a magnetic recording medium is desired to be reduced in its heat shrinkage ratio, and heretofore, a nonmagnetic support having a low heat shrinkage ratio has been used in a magnetic recording medium for the purpose.
For instance, Japanese Patent Provisional Publication No. 59(1984)-11531 discloses a magnetic recording medium having a heat shrinkage ratio of not lower than 1 % in the direction along which the highest heat shrinkage is given when the medium is allowed to stand at 100.degree. C. The publication describes that in the case of using a stretched polyethylene terephthalate film which is widely used as a flexible support (nonmagnetic support), the film is advantageous; further subjected to heat treatment to reduce the heat shrinkage ratio. That is, such heat treatment can reduce the heat shrinkage ratio of the resulting support to not higher than 1%, while the polyethylene terephthalate film prepared by a conventional process generally has a heat shrinkage ratio of approx. 2.3%.
However, when the polyethylene terephthalate film is reduced in the heat shrinkage ratio by applying the above mentioned heat treatment, etc., its Young's modulus generally decreases simultaneously with the reduction of the heat shrinkage ratio. A magnetic recording medium having a flexible support of such low Young's modulus is liable to be deformed in the normal running procedure because of unsatisfactory mechanical or physical strength.
Accordingly, there are restrictions in the preparation of a magnetic recording medium having both of a satisfactory running property and a low heat shrinkage ratio by the conventional methods from the viewpoint of enhancing the mechanical strength of the resulting recording medium.